The invention described herein arose in the course of, or under, Contract No. DE-AC03-SF00098 between the United States Department of Energy and the University of California for the operation of the Lawrence Berkeley Laboratory. The Government may have rights to the invention.
1. Field of the Invention
This invention relates to electroluminescent devices and a method of making same. More particularly, this invention relates to electroluminescent devices formed using one or more layers of semiconductor nanocrystals as an electron transport layer, and preferably capable of emitting voltage-dependent visible light of various wavelengths.
2. Description of the Related Art
Electroluminescent devices such as light emitting diodes (LEDs) have been constructed in the past using P-doped and N-doped materials. However, such devices are generally only capable of emitting color of a particular wavelength based on the semiconductor materials used in the diode. Electroluminescent devices have also been made using a polymeric material such as poly-(p-phenylene vinylene) (PPV) as a hole transport layer between a hole injection electrode and an electron injection electrode. However, such devices are also limited to emission of a single color, based on the type of light emitting polymeric material utilized. Thus, to vary the color, one must use a different polymer, which prevents, or at least complicates, the display of light of various colors. Furthermore, since such polymeric materials do not function as electron transport media, the recombination of holes and electrons, which results in such light emission, occurs adjacent the electron injection electrode, which tends to lower the efficiency of the device as a light source.
An electroluminescent device which addresses this low efficiency problem has been proposed by Brown et al. in an article entitled "Poly(p-phenylenevinylene) light-emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement", published at Appl. Phys. Lett. 61(23) pp. 2793-2795, 1992. Brown et al. teach a device wherein a hole transport layer comprising PPV is separated from the electron injection electrode by an electron transport layer formed using a 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (butyl PBD) molecule dispersed in a poly(methyl methacrylate) (PMMA) polymer. While such a device might provide a higher efficiency than conventional polymeric light emitting devices, it still is limited to the emission of light of a single wavelength by excitation of the polymeric material comprising the hole transport layer.
Tang et al., in an article entitled "Electroluminescence of doped organic thin films", J. Appl. Phys. 65 (9), 1 May 1989, pp. 3610-3616, teaches a device that emits light at different wavelengths by doping an organic transporter such as 8-hydroxyquinoline aluminum with a dye molecule. Light, however, is still limited to emission at the wavelength of that dye molecule. Furthermore the processing technology taught by Tang et at. requires evaporation of the organic molecules, which may be difficult.
It would, therefore, be advantageous to provide an electroluminescent device capable of emitting light of various visible wavelengths in response to external stimulus. More particularly, it would be advantageous to provide such an electroluminescent device wherein variations in voltage would result in change of color of the light emitted by the device. It would be particularly advantageous if the light emitting material was an inorganic material capable of withstanding higher temperatures than the conventional organic polymeric materials.